A sweaty morning at Thorpe Park, and the smell of sunblock and ketchup hangs heavy in the air. In the shade of Saw, a freefall rollercoaster based on the torture-porn franchise, and beside a grey but warming lake, a crowd is gathering by the Embarrassing Bodies truck. For one day only, Dr Christian Jessen and Dr Dawn Harper will be consulting in the back of their well-lit van, a televised surgery open to anybody passing between rides. Provided, of course, that they're within the 70% of the population suffering from an embarrassing illness – varicose veins, excess hair, stretchmarks, alopecia, IBS, obesity. Something that oozes, preferably. Something swollen.

Rosie and Kelly are 13 years old, and so excited to be in the presence of Dr Christian that they're quivering, visibly. As fans of Embarrassing Bodies, the Channel 4 show that offers contributors lengthy medical attention in exchange for a close-up of their glittering piles, they're recalling their favourite episode from the three series so far. Was it the episode with the interior designer's oversized labia? Was it the one about the woman with the udder-like breasts? The one with Christina's anal warts? They remember all of those, but their favourite was the episode where Dr Christian stood in a locker room to compare the penis sizes of a whole rugby team. "It was brilliant," they say. "He was brilliant. I'd definitely go on the show, if I had something wrong." Then I ask Rosie and Kelly, as I will ask many people over the next few days, the question most asked about Embarrassing Bodies: if the problem embarrasses them, why do patients choose to go on telly with it? They answer, but I'll come to that.

Embarrassing Bodies, produced by Maverick TV, first appeared on Channel 4 in 2007, an explosion of incontinence and skin rashes, with Ashley Jensen's voiceover explaining the statistics around the illnesses shown. In the Evening Standard, Victor Lewis-Smith wrote that the show was "admirable, unpalatable, fascinating and repulsive in roughly equal measure". Since then it's become furiously successful, the most watched programme on Channel 4 this year, consistently winning audiences of up to four million, double the average ratings for its 9pm time slot.

The show has covered 120 different conditions to date. After last month's special episode, about a nine-year-old girl called Charlotte who visited the clinic with extreme verrucas – her feet were covered in molluscs – and was found to need a bone marrow transplant, the Anthony Nolan Trust reported a 4,000% increase in enquiries. Embarrassing Bodies's Bafta-award-winning website is responsible for 42% of Channel 4's web traffic, and its STI checker has, at last count, been used by one million people. More than 150,000 viewers took their online autism spectrum test, creating the world's largest test of its kind.

The website is also used as an example of how television is moving towards a "multiplatform" future – spikes in Embarrassing Bodies's web traffic during the show proved viewers are watching with their laptops open, tapping their own questions into the message boards, clicking through the "vulva gallery" or applying to be on the show. Adam Gee, cross-platform commissioning editor for factual programming at Channel 4, explains, "It was the first evidence that multiplatform for factual could work and find an audience."

Along with Drs Dawn Harper and Pixie McKenna, Dr Christian Jessen is the face of Embarrassing Bodies. The face, the arms, the muscular chest, the skin of which can often be seen peeking from a carelessly buttoned polo shirt, the colour of cured meat and the size of a healthy bottom. We meet as he comes off the rollercoaster Colossus, where he's been testing his heart rate for a piece about stress levels. He happily poses for photographs for fans and gives me an apologetic grin as he signs another autograph.

Last year Jessen hit the headlines when Gordon Brown's spin doctor, Damian McBride, sent an email to Labour blogger Derek Draper, suggesting they spread the unfounded story that David Cameron had suffered from a sexually transmitted disease. He also suggested "inserting [a] picture of Dr Christian Jessen", implying the Embarrassing Bodies doctor, also a practising Harley Street GP, had treated the now Prime Minister.

"I can't say I've never met David Cameron," Jessen said at the time, also unable to deny it. "It's interesting, because if you say, 'No, someone's not a patient', is that also a breach of confidentiality?" His main gripe with the story, he said, was the suggestion that treatment for an STI could have brought down a government. "I was particularly disappointed because there I am, slogging away, trying to make people feel more comfortable talking about these things, and then this idiot undoes it all in one email by implying it is shameful and embarrassing. It made me very cross."

Jessen is very posh, very clever and very charming. He grew up in Fulham, west London, went to boarding school and graduated from UCL in 2000, when he moved to Kenya and Uganda to research HIV and malaria in children. Now 32, he lives with his partner and miniature Pinscher in London.

When he joined the show, he didn't think it would last. "I didn't think piles and verrucas would be exciting to a Channel 4 audience," he says, "but I soon realised that people hadn't seen the novelty of haemorrhoids before, because we're usually pretty crap about talking about this stuff. Yes it's a bit gross, but we never treat it in a sensationalistic way. It's good, practical telly, and that's why it works." I can't help but comment on his muscles. They're glinting in the sunlight, swelling like hams in my sightline. He thanks me. "I like it when people think I don't look like a doctor – that's why we were cast. We're not bow-tied, spectacled, dull. There's a need for normal-looking people in the medical profession I think, a crying need for doctors who are approachable." On screen, they're referred to as Dr Christian, Dr Dawn. "It's a very closed business, very secretive and reverential. And our harshest critics are other doctors."

Dr Dee Dawson, medical director of the Rhodes Farm eating disorder clinic, has watched Embarrassing Bodies from her London sofa. She says the show "sensationalises serious illnesses and pleases ghoulish audiences with its tabloid format". But, she continues, "in doing that, it also alerts them to potential problems, and I'm sure people are more likely to see their doctors after watching an episode relevant to them, in the same way that we saw a spike in smear tests after Jade Goody talked about her cervical cancer. It's horrendous," she says, "and so overly dramatic, but it's a positive thing, in the end. In fact, my father only had a melanoma diagnosed after watching a medical programme on TV."

Inside the Embarrassing Bodies truck, a screen separates the surgery set and a tiny waiting room, where a bowl of pink orchids erupts on the coffee table. The first patient to walk in from the park is 23-year-old Natasha, who wants to talk about irritable bowel syndrome. Her boyfriend, Peter, waits patiently by the fence. "We love the show," he tells me. "My mother died of skin cancer this year and the programme showed me the warning signs to look out for. Plus all the blokes with their tackle out – they ask questions I wouldn't dare!"

As the cameras roll, Dr Christian asks Natasha about her loose stools. "The message," he says, after a brief conversation, "is don't panic, but be insistent. Fight for help, and find a GP that will ask for more tests."

Outside in the sunshine Natasha is elated. "That was so fantastic," she says. "He gave me the confidence to go back to my doctor. And yes, it's an embarrassing problem, but when you finally talk about it you feel so much better."

Why did you decide to go on TV with it?

"Well," she says, "I love

the show. And I love that the doctors make themselves so approachable. GPs, usually, are so busy they don't have time to listen. And I honestly think the show is changing young people's views on how they communicate about their bodies."

Rosie and Kelly, still lurking for another glimpse of Dr Christian, agree. They say that their friends wouldn't laugh at them if they appeared on Embarrassing Bodies because the fact it's on TV legitimises the problem – takes the shame away.

Dr Dawn Harper, who's an NHS GP in Gloucestershire when not filming, says one of the reasons people decide to take their problems on the show is the brand recognition. "They've seen us on TV, so they trust us. Plus, there's a huge sector of society that thinks: 'If it's not life threatening, I mustn't bother my doctor', or they've felt a lack of sympathy from them in the past. And while women are always registered with the doctor for their smear test, men are often still registered at their mum's GP. Viewers feel like they know us a bit. They've seen how we interact with patients so they know what to expect. And we have support from more and more specialists as the show grows, so there's the availability of things that aren't offered on the NHS."

I hear all Harper's reasons repeated by patients as the day goes on.

Kelly Coulter, who's brought her 18-month-old son to the truckstop to talk about a problem with his gums, says she'd "absolutely get my breasts out on the show if I was guaranteed a boob job". Plastic surgery is a subject often broached in the programme. Jessen (who talked to the Daily Mail about his hair transplant in May) tells me, "I'm for plastic surgery, as long as the industry doesn't take advantage and prey on us."

One of the surgeries Jessen recommended on an episode in 2008 was a patient's labioplasty. In her book Living Dolls, Natasha Walter details how uneasy this made her feel. "[In this episode] a young woman consulted a doctor about the fact that her labia minora extended slightly beyond her labia majora and that this caused her embarrassment. Instead of reassuring her that this was entirely normal, the doctor recommended, and carried out, surgery on her labia. The comments left on the programme's website showed how this decision to carry out plastic surgery to fit a young woman's body to a so-called norm made other young women feel intensely anxious. 'I'm 15 and I thought I was fine, but since I've watched the programme I've become worried, as mine seem larger than the girl who had hers made surgically smaller! It doesn't make any difference to my life, but I worry now that when I'm older and start having sex I might have problems!' one girl said.

"This idea that there is one correct way for female genitals to look is undoubtedly tied into the rise of pornography… If the rise of pornography was really tied up with women's liberation and empowerment, it would not be increasing women's anxiety about fitting into a narrow physical ideal," wrote Walter.

I ask Jessen whether this patient's referral was a difficult decision. "It's a hugely controversial subject, but she was having dreams about cutting off her labia. To me, that justifies the treatment – she was grossly psychologically disturbed." Is he concerned that with this decision he might encourage female viewers to seek surgery when there's no medical problem? He sighs. "It's our job to show all available treatment. And if it creates a forum for girls to talk about their bodies… We try our best – there's no malice in what we do."

A few days later, in their Birmingham clinic/studio, a set lit so whitely it feels like a 1960s vision of heaven, I meet John. I'd heard about John. Once, over lunch, a friend told me about the programme she'd watched the night before. Wide-eyed, she used her hands to describe an operation they'd shown, where, in order to cure the man's perianal abscesses, they'd stretched his anus open with metal pegs, so that, square, it filled the TV screen neatly. The image stays with her still. When John, now 27, returns to Embarrassing Bodies, it's after 15 operations; he says he's feeling "the best I have in four years".

The problem began when he was 23, and a prison officer in Nottingham. One night, an inmate threw a television from the floor above – it landed inches from where John was standing and he had a panic attack. He took a week off due to stress and it was then that he noticed the first swelling. When the abscess burst inside him, it spread. The pain, he says, was like "being kicked in the spine", and once it caused him to black out. But worse than the pain, he says, was the humiliation of asking his girlfriend, who he'd met while recovering from his eighth operation, to regularly clean his seeping wounds. "He'd had to wear a nappy in his pants," Dr Pixie McKenna tells me. "It was so, so sad."

The episode of Embarrassing Bodies which concentrated on Charlotte Wilson's verrucas inspired John to seek them out – John chased their truck stops around Britain, finally getting a walk-in appointment with Dr Pixie when he'd just come off a night shift.

"I saw that they'd saved that little girl's life, so I wanted them to fight my corner," John tells me. "And I thought if they could help me out, then showing my backside on national TV would be a small price to pay." When John's episode aired, he found out later, his prison colleagues had thrown an Embarrassing Bodies party, where, during the screening of his operation, someone had thrown up on their pepperoni pizza.

Today, after shyly admitting that his girlfriend has agreed to marry him, he's happy to drop his trousers and show his scars to Dr Pixie, who gasps with pleasure. "That is not the same bottom that I first met!" she cries.

Michelle, 45, is birdlike, and woke at five to drive down from Lancaster, where she's proud to live in an Area of Outstanding Natural Beauty. When she told her 19-year-old son she was thinking of approaching Embarrassing Bodies about her constipation after a post- hysterectomy prolapse, he said he wasn't happy about it, but that if it would help, he'd support her. Sitting in the surgery, camera angles mean she must answer Dr Pixie's question – "Do you get any soiling?" four times. Her examination is over in seconds – a rectocele is diagnosed – and the screen where I'm sitting fills with an internal HD image. "Embarrassing Bodies doesn't believe in pixilation," the executive producer, Steph Harris, tells me. "It implies shame."

I talk to Michelle over a cup of tea. "I was very apprehensive, yes," she says. She's sitting up straighter than she was before she entered the surgery – she seems to take up more space, somehow. "I was especially scared about the young cameraman, but he made me feel completely at ease. There was no pressure and lots of explanation. I'm so happy to have come."

But why choose to broadcast your problem, I ask. "I come from a nursing background and this is one of my favourite shows, so I knew I'd get an honest consultation. And if nothing else, I wanted to highlight how common this problem is to other women. I'm not the only one of my friends to watch it either – I have a friend who's in hospital today after diagnosing herself with carpal tunnel syndrome when she saw it on the programme."

For all the reviews pointing out Embarrassing Bodies's "crypto-pornographic nature", the criticism of the show's relationships with cosmetic surgeons, and the format, which relies on suspenseful ad breaks and the promise of genital close-ups, it's the patients who convince me that the programme does good, helping viewers talk about worries they'd otherwise hide.

Dr Dawn tells me about a recent event she went to with her NHS colleagues. "They tease me, of course they do, but that night 40 out of the 60 doctors there told me they'd seen patients purely off the back of the show."

Interestingly, I think this is the only show in the makeover genre to include both male and female contributors. And while the occasional woman's saggy belly is hacked away by surgeons, the overall message is one of practical medicine: less about how you feel about your body, more about the body itself. And it's the only programme on telly where the haemorrhoid's the star.

Embarrassing Bodies series four begins on C4 on 17 September. Can I Just Ask? (Hay House, £12.99) by Dr Christian Jessen, a collection of questions doctors are asked when off-duty, is published on 1 November

Charlotte's storyThe case which turned the show into a national treasure

In the autumn of 2008, nine-year-old Charlotte Wilson's mother, Sofia, led her into Dr Christian's clinic, seeking help for her verrucas. When he saw her feet, he was briefly speechless, but viewers saw his brain whirring into action. Verrucas, a common viral infection which most people's immune systems can quickly defeat, covered her toes in a terrifying crust. Dr Christian realised that this was an indication that Charlotte's immune system was losing the battle. She was referred to Great Ormond Street Hospital where the diagnosis was confirmed. The immunologist warned that in time Charlotte would be vulnerable to life-threatening infections. She needed a bone marrow transplant. Luckily, blood tests revealed that her sister Isabelle was a perfect match. Charlotte received chemotherapy in an isolation room, and after months away from home, she celebrated her 10th birthday in hospital, waiting for the bone marrow to take root.

When the show aired, the Embarrassing Bodies website encouraged viewers to sign up for the Bone Marrow Register and the following day the Anthony Nolan Trust received 1,400 requests for information, as opposed to the usual 30. This week, the show's producer received an email from Sofia Wilson. "Charlotte looks fantastic," wrote her mother. "Lots of curly hair and a suntan... [Recovery] is going to take longer than we thought (Charlotte has to learn to stick a needle under her skin so that she can do her gamma-globulin infusion herself), but you saved our daughter's life".

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